1. Field of the Invention
The present invention relates to the preparation of crystallized aluminum and sodium acid phosphates having the following general formula: EQU Na.sub.a Al.sub.b H.sub.c (PO.sub.4).sub.d . n H.sub.2 O
wherein a, b, c, d and n are numbers respectively ranging from 0.6 to 3.3; 1.8 to 3.3; 12 to 16; 7 to 9; and 0 to 5, with the proviso that a+3b+c=3d.
2. Description of the Prior Art
The acid phosphates of aluminum and sodium (SAP) are compounds well known to this art which in recent years have found widespread applications in the food industry.
They are used, in particular, as delayed action baking powders for a certain number of bakery and/or pastry products, as additives to control the melting of cheeses, and as additives for the fixation of fats in meats.
The compound having the formula of NaAl.sub.3 H.sub.14 (PO.sub.4).sub.8.4 H.sub.2 O is presently the one of greatest industrial importance, as it has been recognized and approved for many years as the common acid baking powder in leavened dough.
To date, two principal processes are known for the preparation of crystallized SAP.
The first, described particularly in U.S. Pat. No. 2,550,490, is a process of the discontinuous type, designated the "methanol process". It entails concentrating and then crystallizing a solution of the precursor of the desired SAP until a highly viscous slurry containing SAP crystals is produced, then separating the crystals from the crystallization medium by filtration. A resuspension is first carried out by the addition of methanol.
This process exhibits two major disadvantages. On the one hand, it requires methanol, which is a highly flammable and toxic compound requiring subsequent purification stages, and, one the other, powerful and very expensive agitating equipment must be used, in view of the extreme variations in the viscosity of the medium over the course of the operation.
The second process, described in U.S. Pat. No. 3,311,448, is based on continuous crystallization using separate zones for the reaction and the subsequent crystallization of the precursor solution.
In actual practice, these two stages are carried out continuously in a conveyor-kneader type apparatus maintained at elevated temperatures, with the precursor solution entering at one end of the apparatus and the crystallized SAP exiting at the other end.
Unfortunately, the industrial application of this process requires a great investment relative to the apparatus, which is voluminous, expensive and consumes much energy, which obviously is a severe economic restraint militating against the use of this particular process.
Furthermore, both of the aforementioned processes have the disadvantage of requiring delicate controls, and consequently produce final products of constant quality only with extreme difficulty.
By "quality" are principally intended, on the one hand, the stoichiometry (and more particularly the number of molecules of the water of crystallization) and, on the other, the degree of crystallization of the final product.
The latter parameter is, among others, of great importance relative to the properties of the SAP for ultimate use. Thus, in a general manner, it is observed that a perfectly crystallized SAP has better delayed reaction properties with the bicarbonate when used as a leavening agent, and, also, that it is very weakly hygroscopic, which eliminates setting of the product mass, which remains a severe industrial problem during the storage and/or handling of the material.
To offset the poor hygroscopic properties (thus, the poor crystallinity) of the SAP produced by the conventional processes described above, it has been proposed to this art to add various stabilizing agents thereto, such as potassium. But this objectionably adds to the overall cost of manufacturing.